As many as 50 Americans and Canadians in Cuba and China in 2016 and 2018 experienced rather sudden symptoms, including impaired hearing, vestibular and memory function, as well as depression and other abnormalities associated with neural function. Initial speculation as to a cause centered on sonic agents, i.e., sounds of some sort, including sub- and ultrasonic sources. The Hearing Journal’sApril 2019 cover story probably provided the best overview of what was known about these “attacks” as of April 2019 (The Hearing Journal. 2019; 72:22,23,26). The author wrote: “So, the puzzle sits, uncompleted. And after two years, pieces are still missing, leaving the possibility of a satisfying solution beyond the grasp of observers.” The article also quotes my previous letter to the editor published in The Hearing Journal (The Hearing Journal. 2019; 72(2):27): “Until more is known,” I said in my letter, “I doubt that even the most erudite speculations will turn out to fully explain the mysteries surrounding these sonic attacks.”
We now know more, but the new information probably only adds new pieces to the puzzle, expands the questions about these events, and fuels additional speculation. The National Academies of Sciences (NAS) has formed a committee to review these events: the Standing Committee of the National Academies of Sciences, Engineering, and Medicine to Advise the Department of State on Unexplained Health Effects on U.S. Government Employees and their Families at Overseas Embassies.
Those testifying before the NAS Committee include all of the lead investigators of the studies published to date and briefly described in The Hearing Journal’s April 2019 cover story, as well as a new study (published online in fall 2019) from a Canadian team. This recent multi-institute (including the School of Communication Sciences and Disorders, Dalhousie University, and Nova Scotia Hearing and Speech Centres), and multiauthored, non-peer-reviewed study (doi: https://doi.org/10.1101/19007096) measured multiple hearing, vestibular, brainstem, and related functions in 23 Canadians who were exposed in one way or another (i.e., the Canadians who reported symptoms and were in Havana, Cuba, close to the time of testing or remotely in time relative to the testing). The main measurements were related to brainstem function. The researchers concluded: “We report clinical, imaging, biochemical, and toxicological evidence consistent with the hypothesis of overexposure to cholinesterase inhibitors as the cause of brain injury in our cohort. While other causes cannot be ruled out, our findings point to an environmental (and possibly insecticidal) risk with immediate implications for prevention, screening, and follow-up of individuals in the context of exposure to such neurotoxins.” The study pointed out that Cuba had an extensive mosquito-spraying program, which was intensified close to the time of the reported attack to deal with the Zika virus. Some of the measures obtained in the study were consistent with a neurotoxic effect that could be due to the ingredients in the pesticides used in Cuba. As I suggested in my previous letter to the editor, such neurotoxins could affect the single arterial system that serves the brainstem and the inner ear.
So a sonic cause is becoming less and less likely. However, as Friedman, et al., correctly noted, “Other causes cannot be ruled out.” More data and less speculation are needed. The first meeting of the NAS Committee was on Dec. 18, 2019, and the second meeting on Feb. 24, 2020. The committee has not published any reports to date, so the next evidence related to this on-going mystery will probably come when the NAS Committee issues its report sometime later this year.